Alkyl esters of fatty acids are important as raw materials for fat and oil products, for example, various surfactants and the like, in oleo-chemical industry. Therefore, the preparation process of the alkyl esters of fatty acids is one of the most important processes as upstream processes in the oleo-chemical industry.
On the other hand, since alkyl esters of fatty acids can be used as biomass-derived diesel fuels, the alkyl esters have been remarked as a new energy source for a substitute to petroleum oil.
Generally, an edible oil which has been disposed from restaurants, food manufacturing factories, households, and the like (waste edible oil), is processed by a method such as a waste edible oil is buried in soil after treatment with a coagulation agent, a waste edible oil is disposed as domestic waste, and incinerated. However, in recent years, a trend for an effective reuse of even a waste edible oil has started to be activated with growing awareness of clean global environment. As one of the proposals, there has been tried to start preparing an oil suitable for a diesel fuel by obtaining a methyl ester of a fatty acid through a transesterification reaction with methanol.
Specifically, a methyl ester of a fatty acid from a vegetable oil used as an edible oil is similar to gas oil in terms of properties such as viscosity or specific gravity, and combustion properties. Therefore, potential use of a methyl ester of a fatty acid as a bio-diesel fuel which can be used in engines without remodeling has been pointed out for ages. Recently, a methyl ester of a fatty acid has been in the spotlight as recyclable bio-fuel, and has been widely used, especially in the U.S. and Europe.
In the U.S. and Europe, however, methyl esters of fatty acids derived from fresh edible oil are mainly used, thereby making it more costly than the gas oil. Therefore, the methyl esters of fatty acids are mainly used in a mixed system with a gas oil. Since the edible oil contains a free fatty acid in an amount of about 3% by weight, it is necessary to subject the edible oil to an alkali treatment to remove the free fatty acid in the form of a fatty acid soap, thereby making the edible oil expensive. Alternatively, even when the waste edible oil is used, the waste edible oil undergoes deterioration and usually contains a free fatty acid in an amount of from about 0.5 to about 2% by weight. In any case, it is necessary to solve this disadvantage associated with the free fatty acid in order to use an inexpensive raw material fat or oil. According to a conventional technique, an alkyl ester of a fatty acid is prepared as a fatty acid soap by using a homogeneous alkali catalyst such as NaOH or KOH at the expense of yield and the catalyst, which is one of the causes of high cost (see, for example, Patent Publication 1).
In addition, according to a conventional technique, the homogeneous alkali catalyst is almost entirely contained in a by-product glycerol. Therefore, a tremendously large cost is necessary for the refining of the glycerol in order to make effective use of the glycerol.
The alkyl ester of a fatty acid obtained after the reaction also contains trace amounts of an alkali and glycerol, and an excellent-quality fuel cannot be obtained unless the alkali and glycerol are removed. For this purpose, it is a general practice to wash the alkyl ester with a large amount of water; however, the washing involves complicated procedures and wastewater treatment is also costly.
As described above, in the conventional technique, there are no methods for preparing alkyl esters of fatty acids for diesel fuels using inexpensive raw materials at low processing cost.    Patent Publication 1: JP-A-Hei-6-313188